The testing of highly aspheric optics often requires complex test arrangements: these test systems can be multi-element and will have both fabrication and alignment errors present in the test wavefront. It may not be feasible to calibrate such systems with conventional optical shop practice. The use of diffractive imitator optics, with carefully controlled fabrication uncertainties, can be used to characterise these systems. We describe the use of reflective imitator CGH optics as calibration artefacts in the calibration of an optical test system used to test ELT primary mirror segments. The optical test system is designed to have two operational modes: one to measure a spherical reference optic; and one to measure the primary mirror segment. The use of diffractive imitators in this test system is designed to provide traceability between these two operational configurations, to quantify residual alignment aberrations, and to quantify fabrication errors in the test system. We outline the design of the optical test system, the design of three imitator CGH artefacts required to provide traceability between the two optical test modes, and our calibration approach. We demonstrate the calibration performance achieved with this approach. Without the use of these imitator artefacts, the absolute accuracy of the optical test is estimated to be 149 nm RMS wavefront, of which 47 nm RMS is attributed to midspatial wavefront errors and 141 nm RMS is attributed to alignment and prescription errors. The repeatability of this calibration has been established as better than 3 nm wavefront standard deviation, with an absolute accuracy of 19 nm RMS wavefront.
We report on the first-ever demonstration of grinding and polishing full-size, off-axis aspheric, mirror segments as
prototypes for an extremely large telescope, processed entirely in the final hexagonal shape. We first describe the overall
strategy for controlling form and mid spatial frequencies, at levels in the vicinity of <10nm RMS surface. This relies first
on direct CNC grinding of the base-form of these 1.4m segments, using the Cranfield BoX™ machine. The segments are
then mounted on a custom designed (Optic Glyndwr Optoelectronic Engineering Group) three segment hydraulic
support, and CNC polished on a Zeeko IRP 1600 machine using a variety of custom tooling. We overview the fullaperture
and sub-aperture metrology techniques used to close the process-loop and certify quality, all of which operate
with the segment in-situ on the IRP1600. We then focus on the pristine edge-definition achieved by the combination of
tool-lift and smoothing operations; results never previously demonstrated on full-size pre-cut hexagonal segments.
Finally, the paper discusses the feasibility of scaling the process to deliver 931 segments in seven years, as required for
the E-ELT project.
This paper addresses two challenges in establishing a new process chain for polishing hexagonal segments for
extremely large telescopes:- i) control of edge and corner profiles in small-tool polishing of hexagons, and ii)
achieving the required smoothness of the bulk aspheric form. We briefly describe the performance of a CNC-grinding
process used to create the off-axis asphere, which established the input-quality for subsequent processing. We then
summarize processes for smoothing ground mid-spatials and pre- and corrective polishing using Zeeko CNC
machines. The impact of two cases is considered; i) all processing stages are performed after the segment is cut
hexagonal, and ii) final rectification of a hexagon after cutting from an aspherised roundel, as an alternative to ionfiguring.
We then report on experimental results on witness samples demonstrating edges and corners close to the EELT
segment specification, and results on a full-aperture spherical segment showing excellent surface smoothness.
Following a 'Call for Tenders' by the European Southern Observatory (ESO) a collaboration headed by OpTIC Glyndwr Ltd is producing seven prototype segments for the European Extremely Large Telescope (E-ELT).
Each hexagonal segment is 1.4 m corner-to-corner with a base radius of curvature (ROC) of 84 m and the combination of 984 segments will lead to a primary mirror with a diameter of 42 m. The polishing of the prototype segments occurs in-house at OpTIC Glyndwr using a Zeeko polishing machine, while in situ interferometry is undertaken using a specifically designed optical test tower built above the polishing machine. To confirm the base radius of curvature of the prototype segments an on-machine non-contact profiler is used. The profiler
attaches to the bridge of the polishing machine and is removed during polishing. The design of the profiler is based upon a nanometre optical component measuring machine (NOM) system, originating from the sychrotron optics community. This instrument determines the height at a series of points across the surface through the integration of measured slope data. This paper presents the operation of the OpTIC-NOM as both an on-machine and off-machine profiler. The
accuracy of the profiler both on- and off-machine and its accuracy is compared against similar profilers through its participation in a round robin study. Radius of curvature results are presented for the master spherical segment (MSS), which is a reference optic for the E-ELT segments and highlights a standard deviation on the mean radius
of curvature of 2.5 mm. Finally, a summary of future work is presented regarding the polishing of the prototype segments and the development of a stitching algorithm to produce 3D surface maps from profilometry data.
We describe progress on a novel process-chain being used to produce eight 1.4m hexagonal segments as prototypes for
the European Extremely Large Telescope - a Master Spherical Segment as a reference, and seven aspheric segments. A
new pilot plant integrates a bespoke full-aperture test-tower designed and built by OpTIC Glyndwr, with a Zeeko 1.6m
polishing machine. The process chain starts with aspherising hexagonal segments on the Cranfield BoX™ grinder,
followed by smoothing, corrective-polishing and edge-rectification using the Zeeko CNC platform. The paper describes
the technology and progress, and anticipates how the process-chain is expected to evolve through the seven segments to
increase both process-speed and surface-quality.